Much of the previous work involving liquid crystal (LC) devices has been directed to display devices such as flat-panel displays. While a substantial amount of process has been made in this regard, the light modulators and other devices associated with liquid crystal display devices do not necessarily serve to solve problems in connection with optical communication applications, with which the present invention is primarily concerned. For example, although reduction in switching time has some usefulness in connection with liquid crystal display devices, this factor is of significantly greater importance in optical communication systems. The standard known as the synchronous optical network (SONET) for fiber optics communications specifies that when there is a network interruption, recovery time should be less than 50 milliseconds. Thus, in order to keep a SONET System in operation, optical switches should respond within 50 milliseconds.
One type of previous liquid crystal modulator having relatively rapid switching time is that generally described as a parallel or anti-parallel nematic liquid crystal modulator. However, parallel (or zero degree) and anti-parallel nematic LC modulators have relatively poor contrast (low extinctions ratios). In particular, due to the unidirectional molecular tilt at the cell boundaries, there is significant residual birefringence when the electrical field is applied to the cell which degrades the extinction ratio of the device. For optical communication applications, it is desired to achieve a contrast ratio or extinction coefficient of at least about -25 dB, more preferably about -30 dB, and even more preferably greater than -30 dB and up to -40 dB or more dB. Although the extinction ratio for parallel LC modulators can be improved e.g. by placement of a compensating birefringent polymer, this increases the complexity and, in most cases, the cost of the modulator (and reliability).
Another liquid crystal structure is known which provides relatively high extinction ratios by using a 90 degree twisted nematic (TN) modulator. Twisted nematic (TN) liquid crystal (LC) has been widely used in electro-optic modulators in applications such as flat panel displays, spatial light modulators, and specialized optical image processors. Such devices are generally fabricated to define twist angles of 90.degree. (for conventional twisting TN) or 180.degree.-270.degree. (for "supertwist" nematic (STN) structure). In this context the twist angle is the angle between the direction of the entrance director and that of the exit director.
A typical TN-LC modulator is made by the following process. Transparent electrode indium-tin-oxide coated glass substrates are generally used for the cell walls. They are spin-coated with alignment material, such as nylon or polyimide, and then buffed, such as by rubbing with silk to define a rubbing direction for each substrate, forming LC directions. The two substrates are brought together with the rubbing directions at 90.degree. with respect to one another (where the angles are measured in the same sense as the LC material twist, i.e. calculated in a right-handed manner when the modulator uses an LC material with right-hand twist characteristics and calculated in a left-handed manner when the modulator uses an LC material with left-hand twist characteristics). Liquid crystal molecules between the substrates are switched between two states when an electrical field is applied to the electrodes. The thickness of the liquid crystal cell is designed such that: ##EQU1##
where .DELTA.n and d are the optical birefringence and the thickness of the liquid crystal material, and .lambda. is the operating wavelength, and .alpha. is a proportionality factor for the twist angles, e.g. .alpha.=1.732 for twist angle=90.degree..
Thus, the modulator acts as a switchable half-wave plate that can selectably (in response to application or non-application of the electrical field) rotate the input linear polarization by 0.degree. or 90.degree.. Without the electrical field, the twisted structure wave guides the polarization of the input light to rotate the polarization by 90.degree.. With application of electrical fields, the waveguiding effect is distorted and the polarization is only partially rotated. With the modulator sandwiched between two crossed or parallel polarizers, analog intensity modulation can be obtained. In a TN geometry as described, the tilt angle of the molecules at the boundaries are perpendicular to each other. This is believed to result in substantial cancellation of the residual birefringence thus increasing contrast. Unfortunately, conventional TN modulators are inappropriate for many optical communications applications because of relatively slow response times, being nearly an order of magnitude slower than parallel nematic LC modulators.
Accordingly, previous materials and devices, while useful in many contexts, including liquid crystal displays, have not previously been configured to achieve both the high contrast and rapid switching speed desirable for optical communications applications. For example, the material known as E44 available from E. Merck Industries has a response time of about 65 milliseconds when fabricated into 90.degree. twisted structures (e.g. for telecom applications). Such switching time can be reduced e.g. to less than about 10 milliseconds if a parallel cell is constructed but such a parallel cell does not provide the necessary contrast.
Another important factor that affects switching time is the thickness of the modulator. Switching time is roughly proportional to viscosity and inversely proportional to the square of the thickness: t.varies..gamma./d.sup.2. This would indicate that faster switching is achieved with a low material viscosity and a thinner cell. For optical communication applications operating at infrared (IR) wavelengths (e.g. about 1550 nm) in order to obtain a thin cell, a large optical birefringence (on the order of 0.26) would be needed to maintain the thickness less than about 5 microns.
Accordingly, it would be advantageous to provide a device which achieves both high contrast (such as an extinction ratio greater than about -25 to -30 dB) and rapid switching (such as a recovery time of about 50 milliseconds or less) preferably operating at infrared wavelengths and temperatures in the range of about 20.degree. C. to 40.degree. C.